For the optimisation of the processing of HDPE packaging at two ALPLA sites in Spain, it is planned to implement a polymer sorting stage in addition to the already existing colour sorting stage of the plastic flakes. Prior to the investment decision, systematic technical processing investigations were carried out in order to quantify which qualities can be achieved with which system configurations and parameter settings. A residual fraction from the bottle sorting step was used for these investigations, which consists of mixed-colour HDPE bottles that contain a very high PP content.
This residual fraction, known internally as "Limpio", was subjected to comminution tests with a subsequent feature class analysis in order to investigate the effects of wear on the knives in the comminution units on the subsequent classification and sorting steps and to determine whether the different comminution behaviour of HDPE and the contaminant PP results in enrichment effects in certain particle size ranges that can be used for separation purposes.
It was found that the mass yield of the produced fines <5 mm changes significantly over time due to degradation of the knives. The mass yield of fines with a fresh set of knives of both shredding units is in the range of 31–42%, whereas with a blunt set of knives it only amounts to 25–33%. In order to achieve ALPLA's target of providing a fines fraction of 40–50% by means of screening, a screen cloth with a mesh size of 6.3 mm would be ideally suited.
In the fractional class analysis, the average PP content of the "Limpio" fraction was determined to be 3.2%. In the fraction <2 mm this content is 16.9%, in the fraction <5 mm 5.4%, and in the remaining coarse fraction >5 mm an average of 2.2% PP is detected. The expected enrichment of PP in the fines could thus be confirmed. The enrichment effect of PP in finer particle size classes tends to be more pronounced with fresh knives. With fresh knives, the PP recovery in the fines amounts to 60.1% with a mass yield of 36.5%; whereas with a blunt knife, the PP recovery is 47.6% with a mass yield of 28.4%.
No significant improvement could be achieved in the separate sensor-based sorting of individual particle size classes compared to sorting without prior classification by means of screening. As expected, coarser plastic flakes can be sorted more efficiently, but the differences are rather small. For the fraction <2 mm the achievable sorting quality decreases significantly.